Encapsulate/Transform an Array into an Array of Objects - mongodb

Question

I am currently in the process of modifying a schema and I need to do a relatively trivial transform using the aggregation framework and a bulkWrite.
I want to be able to take this array:
{
...,
"images" : [
"http://example.com/...",
"http://example.com/...",
"http://example.com/..."
]
}
and aggregate to a similar array where the original value is encapsulated:
{
...,
"images" : [
{url: "http://example.com/..."},
{url: "http://example.com/..."},
{url: "http://example.com/..."}
]
}
This slow query works, but it is ridiculously expensive to unwind an entire collection.
[
{
$match: {}
},
{
$unwind: {
path : "$images",
}
},
{
$group: {
_id: "$_id",
images_2: {$addToSet: {url: "$images"}}
}
},
]
How can this be achieved with project or some other cheaper aggregation?

Answer 1

$map expression should do the job, try this:
db.col.aggregate([
{
$project: {
images: {
$map: {
input: '$images',
as: 'url',
in: {
url: '$$url'
}
}
}
}
}
]);

Answer 2

You don't need to use the bulkWrite() method for this.
You can use the $map aggregation array operator to apply an expression to each element element in your array.
Here, the expression simply create a new object where the value is the item in the array.
let mapExpr = {
"$map": {
"input": "$images",
"as": "imageUrl",
"in": { "url": "$$imageUrl }
}
};
Finally you can use the $out aggregation pipeline operator to overwrite your collection or write the result into a different collection.
Of course $map is not an aggregation pipeline operator so which means that the $map expression must be use in a pipeline stage.
The way you do this depends on your MongoDB version.
The best way is in MongoDB 3.4 using $addFields to change the value of the "images" field in your document.
db.collection.aggregate([
{ "$addFields": { "images": mapExpr }},
{ "$out": "collection }
])
From MongoDB 3.2 backwards, you need to use the $project pipeline stage but you also need to include all the other fields manually in your document
db.collection.aggregate([
{ "$project": { "images": mapExpr } },
{ "$out": "collection }
])